<html>
<head>
<script src="resources/canvas_runner.js"></script>
<style type="text/css">
body, canvas { padding: 0; margin: 0; }
</style>
<script type="text/javascript">

const NUM_PIXELS = 1000;
var webGLContext;
// A very simple vertex shader
const VERTEX_SHADER_STRING = `
  attribute vec2 pos;
  void main() {
    gl_Position = vec4(pos, 0, 1);
  }`;
// A heavy fragment shader that calculates sin() 100,000 times per pixel
const FRAGMENT_SHADER_STRING = `
  precision mediump float;

  void main() {
    float x = 0.0;
    for (float i = 0.0; i < 100000.0; i += 1.0) {
      x = sin(i);
    }
    gl_FragColor = vec4(1.0, 0.0, 1.0, 1.0);
  }`;

function createShaderCanvas(vs, fs) {
  const canvas = document.createElement("canvas");
  canvas.width = Math.sqrt(NUM_PIXELS);
  canvas.height = Math.sqrt(NUM_PIXELS);
  const gl = canvas.getContext("webgl");
  const glProgram = gl.createProgram();
  const vertexShader = gl.createShader(gl.VERTEX_SHADER)
  gl.shaderSource(vertexShader, vs);
  gl.compileShader(vertexShader);
  if (!gl.getShaderParameter(vertexShader, gl.COMPILE_STATUS)) {
    console.error(gl.getShaderInfoLog(vertexShader));
  }
  gl.attachShader(glProgram, vertexShader);

  const fragmentShader = gl.createShader(gl.FRAGMENT_SHADER);
  gl.shaderSource(fragmentShader, fs);
  gl.compileShader(fragmentShader);
  if (!gl.getShaderParameter(fragmentShader, gl.COMPILE_STATUS)) {
    console.error(gl.getShaderInfoLog(fragmentShader));
  }
  gl.attachShader(glProgram, fragmentShader);

  gl.linkProgram(glProgram);
  if (!gl.getProgramParameter(glProgram, gl.LINK_STATUS)) {
    console.error(gl.getProgramInfoLog(glProgram));
  }

  gl.useProgram(glProgram);

  const vertexBuffer = gl.createBuffer();
  gl.bindBuffer(gl.ARRAY_BUFFER, vertexBuffer);
  gl.bufferData(gl.ARRAY_BUFFER, new Int8Array([-3, 1, 1, -3, 1, 1]), gl.STATIC_DRAW);

  gl.enableVertexAttribArray(0);
  gl.vertexAttribPointer(0, 2, gl.BYTE, 0, 0, 0);

  document.body.appendChild(canvas);
  return gl;
}

function doRun() {
  webGLContext.drawArrays(6, 0, 3);
  requestAnimationFrame(doRun);
}

window.onload = function() {
  webGLContext = createShaderCanvas(VERTEX_SHADER_STRING, FRAGMENT_SHADER_STRING);
  webGLContext.drawArrays(6, 0, 3);
  doRun();
}

</script>
</head>
</html>
